Would America fight if Iran closes the Persian Gulf to shipping? Confides the Magic 8-Ball: “Signs point to yes.” Presidential administrations of both parties long have reserved the right to use force in the Gulf region when vital diplomatic, economic or military interests are in peril. And they always seem to be in peril in the Gulf.

Nor is the question merely hypothetical. Threatening to bar the Strait of Hormuz, the lone maritime gateway between the Gulf and the larger Indian Ocean, is a standard tactic for Tehran in times of stress. Times such as now, with rockets flying back and forth between Iranian-backed Islamic Jihad militants and the Israeli Defense Forces, and with new U.S. economic sanctions against the Islamic Republic starting to bite. Vows Alireza Tangsiri, commander of the Revolutionary Guard Corps Navy: “In the event of any threats, we will not have the slightest hesitation to protect and defend Iran’s waterway.”

Tehran means to slam the gateway shut.

Whether it can do so is another matter. The assumption that the Persian Gulf and its environs constitute a potential battleground has been a fixture in U.S. foreign policy and strategy since the Carter administration at least. In his 1980 State of the Union address, President Jimmy Carter articulated a newly forceful approach to Middle East affairs. At the time the region was aflame. Revolution had convulsed Iran; militants were holding the U.S. Embassy staff hostage. The Soviet Union had invaded Afghanistan and appeared poised to make further inroads in the Middle East — warranting diplomatic and military countermeasures of increasing vigor.

The “Carter Doctrine” announced in 1980 became a mainstay of U.S. policy. Few invoke Jimmy Carter’s words nowadays, and the Soviet Union is no more. But Carter’s strategic logic remains compelling four decades later.

During the Reagan years, Tehran showed itself willing to attack shipping traversing Persian Gulf sea lanes. Iran and Iraq carried on a “tanker war” as the maritime component of a land war that spanned most of the 1980s. In so doing, each combatant attacked the other’s export earnings, and thus its war-making potential. Washington showed it was willing to use force to defend mercantile shipping bearing precious cargoes of oil and natural gas. The U.S. Navy mounted Operation Earnest Will, reflagging Kuwaiti tankers under the Stars and Stripes and providing them with escort ships to guard against air and missile attack.

The Iran-Iraq War embroiled U.S. naval forces in more than convoy duty. In 1988, for instance, the cruiser USS Vincennes mistakenly shot down an Iranian Airbus during a surface battle against Iranian vessels. Over the years, Tehran has made threats to halt traffic in the Strait of Hormuz a staple of its policy, especially when tensions reached a crescendo over the Iranian nuclear-weapons program.

And today? It’s doubtful — but, judging from history, hardly impossible — that Iranian leaders would try to make good on their threats to merchantmen. Iranian naval forces could never defeat the U.S. Navy in an open-ocean fight. They would not and need not try. Instead the defenders would deploy aircraft, speedboats and submarines toting anti-aircraft missiles, minelayers depositing floating mines in the Strait, and shore-based anti-ship missiles. Such armaments are well suited to assail mercantile and naval vessels that venture within reach.

In all likelihood, U.S. forces would reply in kind. The interests set forth in the Carter Doctrine remain just as irresistible four decades hence — maybe more so. Middle East fossil fuels remain crucial to economic prosperity. The United States maintains close alliances in the region, not just with Iraq but with Gulf Cooperation Council states such as Saudi Arabia, the United Arab Emirates and Oman. Alliances beget commitments. And the region is home to major U.S. military commands. For example, the U.S. Fifth Fleet is based in Bahrain. No U.S. administration could permit U.S. maritime forces to be stranded in the Gulf, cut off from the Indian Ocean and points beyond.

So Tehran might attempt to follow through on its threats, and Washington would order a forcible response if it did. What then? It is possible to close narrow waterways with “asymmetric” forces such as Iran has built up across the decades. Even threatening merchant shipping would drive up insurance rates, and thus the cost to shipowners of doing business, and the cost to consumers of Gulf oil and gas. How long Tehran could keep the gate barred would depend on how steadfast an effort U.S. and allied forces put forth to pry open access.

Iran can make trouble, then, but chances are the U.S. Navy and its allies could force the Strait of Hormuz. But at what opportunity cost? Iran is far from the only problem before American strategists. U.S. forces must contend with contingencies elsewhere around the Eurasian periphery. Task forces policing the Strait of Hormuz cannot face down China’s navy in the South China Sea, or Russia’s navy in the Mediterranean Sea or Black Sea. Perversely, Tehran, a secondary challenger for the United States, can siphon away strength needed to compete with great powers such as China and Russia.

The U.S. military can handle any individual challenge it confronts. As to whether it can handle them all, let’s ask the Magic 8-Ball again: “Reply hazy; try again.”

About this writer: James Holmes is J. C. Wylie Chair of Maritime Strategy at the Naval War College, coauthor of “Red Star over the Pacific” (second edition newly released), and author of “A Brief Guide to Maritime Strategy” (forthcoming this November). The views voiced here are his alone.

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Boats

O Almighty Lord God, who neither slumberest nor sleepest; Protect and assist, we beseech thee, all those who at home or abroad, by land, by sea, or in the air, are serving this country, that they, being armed with thy defence, may be preserved evermore in all perils; and being filled with wisdom and girded with strength, may do their duty to thy honour and glory; through Jesus Christ our Lord. Amen.